Frequency modulation receiver



June 1942- w. VAN B. ROBERTS FREQUENCY MODULATION RECEIVER Filed Aug.51, 1940 2 Sheets-Sheet 1- fiegraducer INVENTOR VViLZZZ'r Van/C 05 912;BY

ATTORNEY June 16, 1942. w VAN ROBERTS 2,286,377

FREQUENCY MODULATION RECEIVER Filed Aug. 51, 1940 2 Sheet-Sheet 2INVENTOR wagbrfigfioberiis ATTORNEY Patented June 16, 1942 UNITED STTSOFFICE 2,286,377 FREQUENCY MODULATION RECEIVER Walter van to RadioCorporation of Delaware B. Roberts, Princeton, N. J., assignor ofAmerica, a corporation 3 Claims.

My present invention relates to reception of frequency modulated waves(FM), and more particularly to novel FM detectors.

The method used heretofore for the demodulation of FM signals has beento pass the signals through a limiter device to eliminate so far aspossible any variations of amplitude, and then to impress the varyingfrequency-constant amplitude signals upon a frequency discriminatornetwork whose output voltage varies in amplitude in accordance with theinstantaneous frequency of the signal, and finally to detect theresulting amplitude modulated signal in the usual fashion and withwhatever non-linear distortion inheres in the particular amplitudemodulation detector employed. In addition to thisdetector distortion thesaid method is subject to distortion which may result from any lack oflinearity in the relation between the frequency modulation of theoriginal signal and the amplitude modulation produced by thediscriminator network. Furthermore, exact tuning of the system is moredifiicult, especially when the modulation is low, than in the case ofamplitude modulated signals, and limiter devices as used in FM receiversup to the present time are less than ideal in their operation so thatthe effects of undesired amplitude modulation on the original signalsare not entirely eliminated.

The present invention provides means for demodulating FM signals withoutany of the aforesaid drawbacks, and is based on the concept of releasinga fixed amount of electrical charge for each signal wave regardless ofits amplitude, the successively released quanta of charge constitutingan average fiow of current which is necessarily and exactly directlyproportional to the wave frequency. Hence the alternating component ofthis current is an exact reproduction of the frequency modulation of thesignal.

It may be stated that it is one of the main objects of the presentinvention to provide a method of, and means for, demodulating frequencymodulated signals which comprises producing from each wave thereof afixed quantum of charge which is substantially independent of the waveamplitude for waves above a threshold value of amplitude, combining saidquanta to form a current having an average value proportional to wavefrequency, and utilizing the alternating component of said averagevalue.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawings in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawings:

Fig. 1 is an explanatory diagram to illustrate the basic operation ofthe invention,

Fig. 2 shows. an embodiment of the invention,

Fig, 3 illustrates a modification,

Fig. 4 shows a further modification,

Fig. 5 i a circuit diagram of an embodiment based on Fig. 4,

Fig. 6 illustrates a modification of the arrangement in Fig. 5.

Before disclosing means for carrying out the method of the presentinvention in actual practice, it may be helpful to explain the basicnature of the invention in more detail by reference to Fig. 1 wherein isshown a simplified and idealized arrangement. The latter, however, isnot suitable in the form shown for the demodulation of signals involvingfrequencies as high as ordinarily used in radio signalling. It is merelyexplanatory in nature. Fig. 1 shows a signal Wave pick-up device, suchas a grounded antenna A from which signal waves are passed through acoil L whose magnetic field attracts or repels the north pole N of apermanent magnet M according as the current through L is passing in onedirection or the other at the instant in question. Thus, the end N ofthe magnet M will be pulled down once during each half wave of signalpicked up by A, and pushed back up by the reverse half of the signalwave. Mag-- net M is centrally pivoted so that each time its north endis pulled down, its south end rises and by means of pawl P moves aratchet wheel R one tooth. It will be seen that provided the amplitudeof the incoming signal wave is greater than the threshold value requiredto operate the ratchet, the wheel R is moved one notch and only one perarriving signal wave, regardless of the amplitude of the wave. Thus, theaverage speed of rotation of wheel R is strictly proportioned to thefrequency of the incoming signal wave. Wheel-R in turn drives a directcurrent generator G through a mechanical coupling device R1, shown indotted lines, which absorbs the intermittency of the motion of wheel Bso that the speed of rotation of generator G is the average speed ofwheel R. The coupling R1 includes a spring S1 and may be of any wellknown construction.

Spring S is stiff enough so that relatively slow variations of theaverage speed of R, which constitute the frequency modulation of thearriving signal wave, are not absorbed. The generator G may be imaginedas of the magneto type whose output current is directly and accuratelyproportional to its speed, so that the output current varies strictlylinearly with the frequency modulation of the signal wave. As the signalfrequency varies about a mean value, the generator output similarlyvaries about a corresponding mean value. Hence, the output current maybe passed through the primary of a transformer T so that a signalutilization device receives from the secondary winding of T only thealternating component of the varying direct current. The lattercomponent corresponds to the frequency modulation of the signal.

In order to carry out the method illustrated by Fig. 1 in a manneradapted to high frequency signalling the escapement mechanism, includingM, P and R, of Fig. l is replaced by an electron discharge devicecircuit adapted to generate a quantum of electric charge for eacharriving wave. Spring S1 is replaced by a low pass filter designed toabsorb wave frequency variations from the current constituted by thesuccessive pulses of charge, while not absorbing the relatively slowvariations of average current which constitute the demodulated signal.No counterpart for generator G is then required as the varying currentproduced by the signal is adapted to energize a loudspeaker, oramplifier, without being converted to a different form of energy.

Fig. 2 illustrates an embodiment of the invention wherein a form ofrelaxation oscillator is used as the escapement mechanism. In Fig. 2 itis assumed that frequency modulated waves of high frequency have beenconverted to FM waves of much lower frequency by the usualsuperheterodyne converter network followed by intermediate frequency (I.F.) amplification. It is also assumed that automatic volume control(AVC), and preferably some limiter action in one of the amplifier tubes,is also employed, although this limiter action is not absolutelynecessary. Thus, the tube V may be considered for the present purposesas a source of FM signals of relatively low frequency and of fairlyconstant amplitude. plifier. These signals are impressed by way of abroadly tuned circuit I upon a relaxation oscillator including aback-coupled tube 2.

Tube 2 has its plate reactively coupled, as at T1, to its control grid6. The tuned circuit l is reactively coupled, as at M1, to g d 6. Thereactive coupling Mz applies signal energy to diode I. The cathode ofdiode I is connected to its anode by a path including resistor 8,condenser 9 and the secondary winding of coupling M2. Condenser 9 is inseries with the secondary winding of M1. The tap 3 is by-passed toground by condenser 10. The plate 5 of oscillator tube 2 is connected tothe positive terminal of potentiometer 4 through the primary winding oftransformer T, the winding being by-passed by condenser H. The secondarywinding of the transformer may feed any desired type of reproducer, oneor more amplifiers being employed between them if desired. Tube 2 isbiased well beyond cut-off by proper adjustment of tap 3 on thepotentiometer.

When a sufficiently strong signal wave emanates from V to drive the grid6 in the positive direction far enough to start any small flow of Tube Vmay be the final I. F. am-

plate current, this increasing plate current reacts by way oftransformer T1 upon the grid to make it still more positive. Thisprocess builds up until the plate current reaches saturation. Then,since it is no longer increasing there is nothing to maintain the highlypositive grid voltage required to maintain the saturated plate currentcondition, so the plate current begins to fall. The process of feedbackis then reversed, and the sudden cutting off of plate current drives thegrid to a high negative potential leaving the plate current entirely cutoff. It might be thought that since the above described cycle of eventsis completed in a very short time, the positive voltage impressedthrough M1 would still be effective to start another similar cycle.This,

- however, is avoided by the insertion of condenser 9 which traps thelarge negative charge drawn by the grid during the time the grid wasdriven to the aforesaid high positive potential.

Thus, after the cycle is completed the grid returns not to its originalpotential, but to a potential so much beyond the cut-off value that nosignal voltage permitted by the assumed AVC and/or limiter action issufiicient to trip" the oscillator again until the excessive bias hasbeen removed. This excess bias may be removed by a simple leak acrossthe condenser 9, the leak being so adjusted that the bias does not leakoff rapidly enough to permit the same positive wave applied to the gridto trip the oscillator more than once, yet letting the bias leak off intime to permit the succeeding positive wave (or the second or thirdetc., succeeding wave) to trip the oscillator. However, in accordancewith one feature of the invention, an improvement over the aforesaidsimple leak across condenser 9 is achieved by connecting diode 1 inseries with leak 8. The diode is so poled as to permit the leakagecurrent to pass, and the applied signal wave voltage is of such polarityas to oppose the said leakage during the half wave period during whichthe signal voltage impressed on the grid is positive. This leakagecontrolling voltage is I introduced by way of mutual inductance M2. On

the one hand it insures that the excess bias produced by the first cycleof the oscillator, which is started by a positive half wave of signalvoltage on its grid, will not leak off during the remaining portion ofthis half cycle. On the other hand, it accelerates the leakage of thisexcess bias during the half cycle of signal wave during which the gridis driven in the negative sense, and thus insures that the oscillatorwill be properly cooked in readiness to be triggered by the nextpositive impulse in its grid.

To recapitulate, the net result of the arrangement is that once, andonly once, per cycle of signal voltage, the oscillator plate currentrises from zero to saturation and falls to zero again as a function oftime. This function is determined by the circuit constants, and issubstantially not affected by the relatively very small magnitude of thevoltage pulse which triggers off the relaxation cycle. Thus, at eachcycle a definite constant amount of negative charge is caused to pass tothe plate from the space within the tube, and the succession of suchquanta of charge constitutes a plate current whose average value isdirectly proportional to the prevailing signal wave frequency. Thiscurrent is passed through the primary of transformer T, across which thesmall condenser II is connected to absorb the wave frequency variation.Hence, the audio utilization device, such as a telephone receiver orother reproducer, is acted upon-onlyby'the alternating component ofthe-plate current which corresponds to the frequency modulation of theincoming-signals. At this point it may be noted that in the-presentsystem it is advantageous to employ a'low value of mean I. F. since fora given frequency swing the variation of the direct current produced bythe liberation of one charge quantum per wave will havean alternatingcomponent which bears thesame relation to the direct current componentas the frequency-swing does to the meanfrequency. However, the I. F.should be chosenhigh' enough so that the minimum instantaneous frequencyis high compared tothe maximum modulation frequency.

Other forms of relaxation oscillators may equally well be employed. Fig.3 shows an arrangement employing an oscillator known in counters forcosmic rays and the like. This known circuit is that portion of thefigure to the left of the dotted line. The two tubes V1 and V2 have theplates thereof connected through resistors I2 and I3 respectively to acommon terminal to which the direct current potential E1 is applied. Thegrid of V1 is connected to the plate of V2 by a parallelcondenser-resistor network I4. The grid of tube V2 is connected bycondenser-resistor network I5 to the plate of tube V1. The PM signalsare applied to the grids through condensers I6 and-I I. The grids oftubes V1 and V2 are connected to the negative-terminal of E1. The signalsource is connected between the junction of condensers I! and IIi-andthe -E1 terminal. The cathodes of the tubes are at ground potential.Potential developed across resistor I3 is tapped off by a slidable tap20, and after transmission through any desired transmission line 2I(shown as a dotted line) is applied to rectifier tube 22 across networkC2R2. The rectifier tube is energized from a direct current voltagepotentiometer 23. The cathode of tube 22 may be adjusted to a point onpotentiometer 23 such that the grid of tube 22 is biased to cut-off.

In this circuit, the constants being suitably chosen, there are twostable conditions; one being with plate current entirely out off in oneof the tubes V1V2 and the current in the other'tubes plate resistorbeing substantially the maximum that can be caused to flow by theimpressed voltage E1. The other'stable'condition is similar, but withthe tubes'interchanged. It has-been found that an input alternatingvoltage will cause a switchover, from one of these two stable conditionsto the other, once andonly once per cycle of input voltage. As a result,the potential at the plate of V2, for example, variesin a somewhatsquare wave fashion between limits determined almost solely by circuitconstants and with a frequency integrally related to that of the inputvoltage. In accordance with the invention, the

potential at the plate of V2 may be impressed by way of line 2| upon theseries combination of C2 and R2.

Now, if the duration of the constant potential portions of the squarewave of potential at the plate of V2 is sufficient to permit conditionsto reach a steady state, a charge equal to the capacity of C11.multiplied by the potential change at the plate of V2 will flow throughresistor R2 at each half cycle of said change. Thus, the time integralof current through R2 per cycle of input voltage is constant. Therectifier 22 is associated with R2 to eliminate the effects of voltagesin one direction. The rectifier is a tubebiased t0 cut-off so thatvoltage drops in one direction cause pulses of. plate current whiledropsinrthe other direction do not. Thus, theaverage plate currentis-di-rectly' proportional to the prevailing frequency of the FM inputsignals. For the purposeof detection. the input voltage is, of course,the amplified, and preferably limited, I. F. signals assumed inconnection with the circuit of Fig. 2.

It is, of course, possible to connect another circuit, similar to thatto the right of the dotted line of Fig. 3, to the plate resistor I2 ofV1, and to combine the outputs of the two rectifiers for the purpose ofobtaining increased output. Also, since the circuit to the left of thedotted line 2| happens to be one designed to give plate potentialalternations in either one of the tubes at half the frequency of theinput voltage, it is possible to utilize the potential variations. ofeither plate to operate another similar system, and so on, so as topermit operating the detector portions of the system at as low afrequency as desired.

Still another type of relaxation circuit suitable for use in myinvention is disclosed in my pending application Serial No. 310,115,filed December 20, 1939. This arrangement need not be discussed. indetail as it will be obvious to those skilled in the art how it may besubstituted for the arrangement shown in. Fig. 2. In like manner it willbe sufficient merely to mention that a switching back and forth betweentwo stable conditions, as employed in Fig. 3, may be achieved in .adynatron circuit wherein'the presence of more than a critical value ofdynode resistance the dynode potential may be swung from a high stablevalue to a'low stable value, or vice versa, in a snap switch manner bythe application of a signal voltage greater than a necessary thresholdvalue in the dynode circuit.

In the arrangements hitherto discussed, the signal voltage hasfunctioned merely to trigger off'some device which then actsindependently of the signal to generate a fixed quantum of charge ateach cycle of operation. In what is to follow, on the other hand, thesignal wave itself generates the necessary fixed quantum of charge percycle by virtue of a limiting action in combination with circuit meansfor deriving a fixed quantum: of charge from a fiat-topped wave. Itshould be emphasized at the outset that a limiter may be entirelysatisfactory for the present purpose, and yet not be entirelysatisfactory for previous methods of FM reception. For example, if aperfectlimiter, (that is, one which chops the peaks off of waves 50 thatthe peaks of the output waves of the limiter are all of equal magnituderegardless of input so long as the input is above a threshold value) isused to feeda resonant system, the voltage developed in the resonantsystem will not be independent of the amplitude of the limiter input.This is true because the fundamental frequency component of the limitedwave is a function not only of its peak value, but, also, of its waveform and hence of its amplitude prior to being limited. In contrast tothis unfortunate. result it will now be demonstrated that a quantum ofcharge may be developed from a flat-topped, limited wave, which quantumis independent of the wave form provided certain precautions are taken.

Fig. 4 shows a screen grid tube arranged to transmit a limited wave ofcurrent to the output circuit; that is, a current whose value is limitedto a fixed magnitude ii for aportion of its cycle, then changes in .anarbitrary fashion to a second fixed value i2 which is held'constant foranother portion of its cycle, then changes back again in an unknownfashion to the first limiting value ii. Let us see what happens when thecurrent changes from i1 to 2. Neglecting for the moment the presence ofthe rectifier D the circuit equation is:

where z' is the primary current, is the secondary current, and is thetime derivative of q. Integrating we have:

his an instant at which i=i1 and IE2 is an instant at which i=i2 ismoved in the secondary circuit, and this quantum is determined solely bythe limiting value of the limited current wave and not .by the wave formaccording to which the current passes from one limiting value to theother.

Of course, this result is based on the assumptions stated, which may bedescribed from a more physical point of view by stating that the timeconstant L/R must be sufiiciently small so that the system attains asteady state condition during the time the primary current i remainsconstant at each limiting value. It is now pointed out that rectifier Dwill prevent any backwards displacement of charge in the secondarycircuit, so that each cycle of the limited voltage will cause a fixedcharge displacement in the same direction through R3. Thus, the averagecurrent through R3 is F a i.)

and the average voltage across R3, is, therefore, FM (i2i1) which isdirectly proportional to the frequency F of the limited current wave.This voltage may then be amplified by a vacuum tube, and the alternatingcomponent of average plate circuit impressed on a loudspeaker.

Fig. 5 shows a complete circuit of the type analyzed in connection withFig. 4. Signal voltage at I. F. is impressed on a diode 3| which permitsonly negative half waves of voltage to occur across resistor 32, i. e.,with a polarity such as to drive the screen grid tube 33 toward cut-off.Resistor 34 helps prevent the grid 35 being driven positive by anycapacity across the diode 3!. Thus, the plate current of this tube has aconstant normal value throughout one half cycle, while it is zero for atime which is variable but is a considerable portion of the other halfcycle for all amplitudes of signal wave above a certain threshold level.The output of the combination of diode 3| and screen grid tube 33, whichconstitutes the limiter shown here as illustrative, is impressed on thecircuit described in connection with Fig. 4. The average voltage acrossresistor 40 is FMz'o, Where in is the normal plate current of the screengrid tube. This voltage across resistor 40 is impressed on a followingamplifier tube 4|, preferably with negative polarity, so as to make anyother bias unnecessary, and the output is utilized to operate areproducer. A small condenser 42 may be shunted across the reproducer toabsorb wave frequency variation of the amplifier plate current. Theplate circuit of tube 33 is coupled as at 50, to the rectifier D. Theanode of the latter is connected by resistor 5| to the grid of amplifier4|,

It will be understood that any other form of limiter may be substitutedfor the one shown, provided that its output current is approximatelysquare wave in nature. Also, an auto-transformer may be used in place ofthe transformer shown in the limiter output circuit, and the requiredrectification of the alternating quanta of charge displacement may beeifected in various ways, including the use of a grid-biased detector.Finally, it should be pointed out that any arrangement operating in themanner of the system shown at the right of the dotted line of Fig. 3may, also, be employed to follow a limiting device supplying a limitedvoltage wave as well as in connection with the generator of squarevoltage waves shown at the left of Fig. 3.

In Fig. 6 there is shown such a combination. Here the limiter 6U,schematically represented, feeds the limited FM signals to the inputelectrodes of screen grid amplifier tube 6|. The plate circuit of tube6| includes a resistor 62. The plate end of resistor 62 is coupled bycondenser 63 to the upper end of resistor 64. The diode 65 is connectedbetween the upper end of resistor 64 and its grounded end. A seconddiode 66 has its anode connected to the anode of diode 65 throughresistor 61, while the anode of diode 66 is further connected to theinput grid of amplifier tube I0 through resistor H. The cathodes ofdiode 66 and tube 10 are at ground potential. The plate circuit ofamplifier tube 10, properly by-passed, may feed-the detected audiovoltage to any desired type of reproducer.

The plate resistor 62 develops a square Wave of voltage, while condenser63 has pass through it a definite quantum of charge at each half cycleof the square wave voltage. The resistor 64 develops a voltage from thepassage of the aforesaid charge in one direction, and amplifier tube 10is energized by this voltage. The pair of diodes 65-65 are arranged incombination with resistors 6'l'l| to form a polarity filter to permitthe transfer of negative potential pulses from resistor 64 to theamplifier, but to suppress positive potential pulses. The amplifieroutput is similar to that shown in Fig. 4. In Fig. 6 the constants areagain so chosen as to permit the system to assume steady stateconditions within the time during which the flat-topped plate potentialwave stays constant at a limiting value. If E is the maximum potentialchange across 62, and F the frequency of the limited wave, then thecharge displacement per cycle is CE (where C is the capacity of 63), andthe average current through resistor 64 is FCE. The average voltageimpressed on the final amplifier grid is FCER, where R is the resistanceof element 64. The rectifying arrangement shown in Fig. 6 has been madedifferent from that shown in Fig. 3 for the sake of variety, as it iswell to emphasize that various types of rectifying circuits may beemployed.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a receiver of the type wherein high frequency waves modulated infrequency to a wide frequency deviation in accordance with audiofrequency modulating voltage are converted to intermediate frequencyWaves of frequency which is low compared to said high frequency, buthigher than the maximum deviation of frequency of said high frequencywaves, means, including a high vacuum electron discharge devicecharacterized by continuous control of its output current by its inputvoltage, for producing, from such of said intermediate frequency wavesas exceed a predetermined amplitude, amplitude limited waves, each ofthe limited Waves having a substantially fixed difference betweenmaximum and minimum values of intensity and each having at least oneextremum of intensity which remains substantially constant throughout alarge portion of the half-period of said limited wave, a resistanceelement, a reactance coupling element for impressing said limited wavesupon said resistance element, the time constant of the combination ofsaid resistance and reactance elements being substantially less than theduration of said constant intensity portion of said limited Wave, meansfor smoothing out current pulses in said resistance produced by changesin intensity of said limited waves to said extremum of intensity fromthe opposite extremum, indicating means for said smoothed out pulses,and rectifying means for preventing current pulses in said resistance,produced by changes in intensity of said limited waves from saidextremum of intensity to the opposite extremum, from affecting saidindicating means.

2. In a demodulator for frequency modulated waves, means to produce in afirst circuit, from said frequency modulated waves, limited currentwaves each having a substantially constant current portion whoseduration is a large portion of the half-period of said waves, a secondcircuit including in series an inductance and a resistance, the timeconstant of said second circuit being less than said duration, mutualinductance means coupling said first and second circuits, a rectifierarranged to be responsive to current in said second circuit, lowfrequency amplifying means for amplifying the output of said rectifier,and an indicating means for the output of said amplifier.

3. A demodulator for frequency modulated waves comprising an amplifier,a high vacuum electron discharge limiting device characterized by anoutput which is a continuous function of the input thereto, connectionsfor impressing said frequency modulated waves on said amplifier, meansfor impressing the output of said amplifier upon the input of saidlimiter device, a first resistance arranged to be traversed by thelimited wave output of said limiter, a condenser and a second resistanceconnected in series with each other, the series combination beingconnected in shunt to said first resistance, the time constant of saidcondenser and second resistance being substantially less than thehalf-period of said limited waves, rectifier means associated with saidsecond resistance and amplifying and indicating means connected to theoutput of said rectifier.

WALTER VAN B. ROBERTS.

